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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a {
            font-size: 2em;
        }

        .a span {
            font-size: .5em;
        }
        .a b {
            font-size: 2em;
        }
    </style>
</head>
<body>

    <!-- 
        em : 相当于父元素的一个字体大小    
     -->



    <div class="a">
        呵呵
        <span>哈哈 <b>呵呵</b></span>

    </div>
</body>
</html>